1. Field of the Invention
The present invention relates to an active matrix display device using thin film transistors (TFTs) and a manufacturing method thereof. In particular, the present invention relates to a method of preventing breakdown of an element due to static electricity and plasma discharge generated in processes and after a test operation.
2. Description of the Related Art
Recently, an active matrix display device in which a peripheral circuit is integrally formed on a substrate has been developed. The active matrix display device is a semiconductor device having a structure in which an active matrix circuit including a display portion formed of a TFT and a peripheral circuit formed also of a TFT are integrally formed on a substrate.
With respect to the manufacture, substrates to be processed have been enlarged for improvement of productivity, and a glass substrate sized 600×720 mm is mainly used in recent years. Enlargement of substrates is considered to be further developed in future. The number of display devices fabricated from one substrate is increased (multiple pattern technology) by the enlargement of substrates to be processed, and the productivity is thus significantly enhanced.
FIG. 12 is an example of a cross sectional view of the TFT. As shown in the figure, a circuit formed of the TFT has a laminated structure. The laminated structure of the circuit can be obtained by sequentially performing the deposition of each layer over a substrate to be processed and forming of desired circuit patterns.
A circuit pattern of each layer is generally formed by photolithography. In performing photolithography, a circuit pattern is exposed to a photo resist deposited on each layer, by irradiating light to the photo resist through a reticle including the circuit pattern. Then, an etching process is performed using the obtained resist film as a protective film to form the circuit pattern.
A step-and-repeat exposure is one of the methods used for forming a plurality of circuits having the same shape and size on one large scale substrate to be processed. FIG. 11 shows a schematic view of an exposure system for performing the step-and-repeat exposure. The exposure system includes a light source 1101 for exposing, blinds 1103 to 1106 for selecting an exposed region over a reticle by partially blocking a light from the light source, a reticle 1102, a projection lens 1108 for projecting a pattern, and a movable stage 1110 for moving a substrate to be processed 1109.
In the step-and-repeat exposing method, the exposure of the photo resist 1111 which is performed in photolithography through the reticle including a circuit pattern, and the moving of the substrate to be processed 1109 are alternately repeated. The photo resist on the substrate to be processed can thus be exposed to light to form a plurality of circuit patterns having the same shape and size.
In such a manner, the exposure is sequentially performed through an exposure unit Dis 105 formed on a reticle 102 and including a display device pattern 103b and a signal input terminal pattern 104b as shown in FIG 1B. Accordingly, a plurality of display devices 103a and signal input terminals 104a can be formed on the substrate to be processed as shown in FIG. 1A.
That is, a circuit pattern of each layer for forming a circuit is formed on the reticle and the substrate to be processed is sequentially exposed to light for each layer. Accordingly, a plurality of desired circuits can be formed by using a small number of reticles, and the production cost of circuits can be reduced.
In the step-and-repeat exposing method, time required for processing a series of exposure becomes generally shorter in accordance with the reduction in the number of exposure. Further, the exposure time becomes shorter in accordance with the reduction in the number of reticles used for the exposure, because the reticles need not be changed many times. In addition, the production cost of reticles can be reduced in accordance with the reduction in the number of reticles used for the exposure.
In manufacturing a display device, forming of a pattern other than the display device is occasionally required to take a measure against static electricity, to perform a test process more efficiently and the like.
For example, as shown in FIG. 2A, a substrate to be processed 101 may be sequentially exposed to light through not only a display device 103a and a signal input terminal 104a but also a circuit group (hereinafter referred to as a pattern A) 201a utilized for the measure against static electricity and for the test process.
In this case, the exposure can be sequentially performed by integrally forming a display device pattern 103b, a signal input terminal pattern 104b, and a pattern A 201b on the reticle 102 to form these patterns simultaneously as shown in FIG. 2B. By using this method, even when patterns other than the display device are formed on the reticle, the exposure time is not prolonged, that is, productivity is not reduced.
FIGS. 3A and 3B show an example of the above-described case, that is, the case where patterns other than the display device are formed on a reticle. As shown in FIG. 3A, a plurality of display devices 103a, signal input terminals 104a, and wirings for short circuit S 301a are formed on the substrate to be processed 101. The wirings for short circuit S 301a are formed by sequentially exposing the substrate to be processed 101 to light through an exposure unit Dis+S 302 formed on the reticle 102 and including a wiring pattern for short circuit S 301b and the display device pattern 103b as shown in FIG. 3B. Therefore, the wirings for short circuit S 301a can be obtained without reducing the productivity of the display device itself.
When using a glass substrate or a quartz substrate as a substrate to be processed, insulating property on the surface of the substrate becomes high. Accordingly, in the process of plasma CVD film forming, sputtering, and dry etching or the like, electrostatic breakdown is observed. Electrostatic breakdown: is a phenomenon in which elements are broken due to the potential difference generated by the charged electrostatic load on the wire free of short circuits. Electrostatic breakdown may also occur by electrostatic generated in an alignment or a transport of the substrate.
As a measure against such electrostatic breakdown, the signal input terminals are conventionally short circuited to each other in processes to prevent the potential difference due to the charged electrostatic load from being easily generated. The wirings for short circuit S 301a, which are formed so that the signal input lines of the display devices are directly short circuited to each other as shown in FIG. 3, are generally removed in a process of separating the display devices on the substrate to be processed into individual devices.
On the other hand, when there are a plurality of patterns other than the display device pattern, e. g., a pattern B 401a, a pattern C 402a, a pattern D 403a, and a pattern E 404a as shown in FIG. 4A, these patterns can be repeated without integrating with the display device 103a. In such a case, two exposure methods described below are performed.
In a first exposure method, provided on a reticle are individual exposure units: the exposure unit Dis 105, an exposure unit B 406, an exposure unit C 407, an exposure unit D 408, and an exposure unit E 409 in which the display device pattern 103b, the pattern B 401b, the pattern C 402b, the pattern D 403b, and the pattern E 404b are respectively formed as shown in FIG. 4B.
In a second exposure method, the same pattern as FIG. 4A is formed on the substrate to be processed 101 by using a plurality of reticles 102 having different structures as shown in FIG. 5A Specifically, provided on each reticle are an exposure unit Dis+B 501, an exposure unit Dis+C 502, an exposure unit Dis+D 503, and an exposure unit Dis+E 504 in which the pattern B 401b, the pattern C 402b, the pattern D 403b, and the pattern E 404b are respectively formed in combination with the display device pattern 103b. 
In the first exposure method as shown in FIG. 4B, not only the exposure of the exposure unit Dis 105 including only the display device pattern 103b but also the exposure of other exposure units (the exposure unit B 406, the exposure unit C 407, the exposure unit D 408, and the exposure unit E 409) is required. Therefore, the exposure time is prolonged.
In the second exposure method, the number of reticles is increased. Therefore, both the exposure time and the production cost of reticles are increased.
Accordingly, in the case of taking a measure against static electricity and performing a test process more efficiently by using an irregular pattern as compared to the device pattern, the productivity is deteriorated in both the first and second methods.
In addition, in a process of separating the display devices into individual devices to achieve the manufacture thereof, after forming a plurality of display devices on a large scale substrate to be processed, circuit tests are required to see whether each display device operates normally. Each circuit has to be independent at this time because the circuit test is performed in the process. Accordingly, the wirings for short circuit S 301a described above need to be cut off before the circuit test.
That is, the wirings for short circuit S 301a no longer operate in processes after the circuit test. It was thus a problem that the conventional wirings for short circuit S 301a could not prevent the breakdown of elements due to electrostatic generated after the circuit test.
It is preferable that there is no short circuit between signal input terminals in the circuit test, and they are short circuited in other processes, in transporting the substrate or the like.
In the process of cutting wires for short circuit before the test, the amount of time required for cutting wires may be shortened by putting together a plurality of cut sections of the wires for short circuit. The cut section is preferably placed on the edge of a substrate for minimizing the effect of particles from the cut section on the display device. In such a case, the wiring pattern needs to be led out from the input terminal to the edge of a substrate to be processed.
However, as shown in FIG. 6, a lead-out wiring 601 which is led out from each input terminal to the edge of a substrate to be processed is arranged irregularly as compared to the display device 103a. The two methods described above must thus be used for forming the lead-out wiring 601.
An exposure pattern as shown in FIG. 7A can be obtained according to the first exposure method. Specifically, the exposure pattern is sequentially exposed to light through a reticle provided with the exposure unit Dis 105, an exposure unit BB 705, an exposure unit BC 706, an exposure unit BD 707, and an exposure unit BE 708 including the display device pattern 103b, a divided wiring pattern BB 701, a divided wiring pattern BC 702, a divided wiring pattern BD 703, and a divided wiring pattern BE 704 respectively as shown in FIG. 7B. Note that these divided wiring patterns are obtained by dividing the lead-out wiring 601.
An exposure pattern as shown in FIG. 8A can also be obtained according to the second exposure method. Specifically, the exposure pattern is sequentially exposed to light through a plurality, of reticles 102 each provided with an exposure unit Dis+BB 801, an exposure unit Dis+BC 802, an exposure unit Dis+BD 803, and an exposure unit Dis+BE 804 including a divided wiring pattern BB 701, a divided wiring pattern BC 702, a divided wiring pattern BD 703, and a divided wiring pattern BE 704 respectively in combination with the display device pattern 103b as shown in FIG. 8B.
In either method, there are some disadvantages such as increase in the exposure time and the number of reticles which affect directly the productivity. As a result, there has been a problem that the productivity is reduced in forming the wiring pattern led out from the input terminals to the edge of the substrate to be processed as compared to in forming only the display device on the substrate to be processed.
In view of the foregoing, the present invention will provide a method for efficiently forming a wiring pattern led out from input terminals, which are formed on the periphery of the display device to prevent electrostatic breakdown, to the edge of a substrate to be processed by using the novel configuration and method.